Infiltration

Infiltration is the process by which a substance permeates into or through a material or area. The term is used in several disciplines with related but distinct meanings. In hydrology and soil science, infiltration refers to the entry of water from the surface into soils or porous media. The rate at which water infiltrates depends on soil texture and structure, moisture, temperature, and land cover. Models such as Horton, Green-Ampt, and Philip describe infiltration behavior; measurements are often made with infiltrometers and rainfall-runoff experiments. In urban hydrology, controlled infiltration is used to recharge groundwater and manage stormwater through infiltration basins, trenches, and permeable pavements.

In building science, air infiltration describes the uncontrolled flow of outside air into a building through cracks and openings. It affects energy efficiency and indoor air quality and is influenced by wind, stack effects, and construction details. Field and laboratory tests quantify infiltration, using devices such as blower doors and pressure sensors.

In medicine and biology, infiltration denotes the movement of substances or cells into tissues. It can refer to edema caused by intravenous infusion leakage (IV infiltration), or to cellular migration during inflammation or tumor invasion. Pathologists may describe immune cell infiltration in inflamed tissue or cancer cell infiltration of surrounding stroma.

In military, espionage, and security contexts, infiltration denotes covert entry or penetration of an organization or territory by agents or operatives to gather information, influence events, or support a mission. It is discussed in historical, strategic, and counterintelligence literature, with attention to legal and ethical considerations.

Across disciplines, infiltration is a general term for permeation from a surface into a domain, with outcomes that range from natural processes to clinical conditions and security operations.